//https://leetcode.cn/problems/peak-index-in-a-mountain-array/description/
class Solution {
    public int peakIndexInMountainArray(int[] arr) {//该数组一定为山脉数组
        int left = 0;
        int right = arr.length-1;
        while (left < right) {
            int mid = left + (right - left + 1) / 2;
            if (arr[mid] > arr[mid - 1]) {
                left = mid;
            }else {
                right = mid - 1;
            }
        }
        return left;
    }
}